A calibration technique for testing a device under test.
The Line-Reflect-Reflect-Match (LRRM) vector network analyzer calibration method with automatic load inductance correction has been an accepted and reliable work horse for on-wafer probing measurement for more than a decade. LRRM is valued for its relative insensitivity to small errors in probe placement that are inherent in microwave probing. Typical LRRM calibrations compare favorably with the NIST reference multiline Thru-Reflect-Line (TRL) method yet require only simple fixed spacing standards using the same set as the Short-Open-Load-Thru (SOLT) method.
In the most common use of the LRRM algorithm, impedance standard substrate standards are positioned to allow probing using fixed spacing probes with minimal spacing, as illustrated in FIG. 1. The Line (or Thru) standard is kept electrically short and the reflect and match standards are situated at the probe tips, approximately co-located with the desired measurement reference planes. This configuration reflects design choices made to minimize impacts from non-ideal or unknown behavior of the Line standard in loss, frequency dependent delay, or impedance match. The configuration also facilitates convenient automation of the calibration using only substrate moves resulting in not only the convenience of a one-button calibration but also enhanced repeatability by avoiding probe repositioning.
As maximum testing frequency has risen to 110 GHz and beyond, the electrical length and inductive reactance of existing, commonly used calibration standards has grown to where these impacts are no longer transparent. This calibration error is not necessarily the dominant measurement error since probe to DUT positioning uncertainty also has greater impact at higher frequency. Also, less frequent but important situations (such as probe card measurement of larger die size) require electrically long lines for the Line standard used in calibration, encountering at even lower frequency the limitations.
An enhanced LRRM (eLRRM) technique for improved handling of non-ideal and electrically long Line standards which uses a more robust load inductance extraction method is desirable.